NullPointerException

Hi,
Can anyone explain to me what "NullPointerException" means Ive thoroughly gone through my code and don't see any errors but after executing one function of my program it gives me 'Exception in thread "main" java.lang.NullPointerException'.

The reason this causes an error is because the object o doesn't refer to anything, it's simple a reference. Calling toString on it causes a null pointer exception since o doesn't point(refer) at anything

For more help please post the pertinent code and EXACT error message(copy/paste). Post code in code tags.